Electrolytic etching of zinc plates



Patented Feb. 5, 1946 s PATENT orrlca ELECTROLYTIC ETCHING 0F ZlNCPLATES Albert R. Kreck, Bel-Wyn, Ill.

No Drawing. Application December 7, 1942,

' Serial No. 468,107

2 Claims.

This invention is directed to a method of electrolytically etching zincwhich subject is dealt with in my co-pending application, Serial No.457,935, filed September 11, 1942. It is particularly applicable to theetching of zinc printing plates where a clean, smooth, even etch is required, and although it is particularly adapted -to meet the exactingrequirements for printing plates, it is equally adapted for theproduction of etched plates intended for other purposes.

Heretofore zinc has been etched by nitric acid. Electrolytic etching mayprove unsatisfactory in that rough, uneven, and spotty surfaces resultfrom the rapid deposition of a film or coating on the metal which retardor obstruct the current flow, or otherwise interfere with the uniformremoval of metallic particles from the exposed areas of the plate whichshould at all stages present a clean, bright metallic surface which canbe etched cleanly and rapidly.

This is particularly true in cases where halftone reproductions are tobe taken from photographs and in which the printing surface is affordedby a multitude of minute dot-like elevations which by their groupingproduce variations in shading and which are produced by the etching awayof the metal exposed in the pattern photographed or otherwise imprintedon the surface of the plate. Unless the etching proceeds with certaintyand precision, blurred and fuzzy effects will result which render theplate useless for printing purposes.

'The present invention pertains\ to the method of etching zincelectrolytically using a moderate voltage, approximately 10 to 30 volts,determined according to size of plates, a cathode at an approximatedistance of two to three inches away from the zinc plate to be etchedand an electrolyte which includes sodium sulphate with the optionaladdition of one or more of the other sulphates recited below, to whichhas been added a limited quantity of ammonium thiocyanate which acts asa carrying agent to speed the current flow as it is a strongelectrolyte.

I am aware of the fact that heretofore zinc plating has been performedelectrolytically, but the problems involved in the etching away of amultitude of minute exposed surface particles to expose in relief theprinting surface are problems of a nature quite at variance with thoseinvolved in zinc plating in which an even deposition of metal on thesurface of the cathode is the end in view and in which the anode servesmerely at the positive pole to replenish the metallic content of theelectrolyte from which the metallic coating is derived and which wouldotherwise become depleted as the metal is withdrawn therefrom anddeposited on the surface of thecathode.

In zinc etching on the contrary the etched condition of the Zinc anodeitself is the subject to which attention must be directed while themetallic particles removed therefrom do not replenish the electrolyte,but rather accumulate as a sludge on the bottom of the tank and which isdiscarded. The cathode serves merely as the negative pole for thecurrent flow and if any of the zinc particles etched from the platemigrate to the cathode they merely adhere thereto as an easily removablecoating which may occasionally becleaned off possibly once a day asoccasion may require and do not afford anything in the nature ofa platedsurface.

In seeking to provide a bright metallic zinc surface at all stages inthe etching operation, I

have found that it'is necessary to employ an electrolyte which willmaintain the plate surface smooth and clean and will have no tendency toform a deposition on thezinc anode of any'kind of a surface coatingwhether in the form of a.

thin soapy film which retards the current flow and requires the voltageto be stepped up, or of a zinc sulphate coating of granular consist encywhich tends to retard the etching speed and prevent an even andcontinuous removal of metallic particles or other scum, coating or oxideof whatsoever character. 1

I have found that electrolytic treatment in .a

solution of sodium sulphate in water will etchthe zinc and carry thecurrent but will result shortly in the deposition of a thin film whichslows down the current flow and quickly retards the etching unless thevoltage be increased.

It thus becomes desirable to employ an additional agent which does nottend to'cause the deposition on the anode of any film, soapy orotherwise, thus keeping the plate clean and bright at all times andmakin it possible to secure reliable and uniform results withsubstantially constant voltage under a given set of. conditions and withless current flow throughout.

I have found that-the addition to the sodium sulphate solution of arelatively small amount of ammonium thiocyanate as a carrying agent willproduce quite satisfactory results although the employment of smallamounts of ammonium sulphate and zinc sulphate in addition to the sodiumsulphate and the ammonium thiocyanate will improve the result where moreexacting requirements are specified.

Satisfactory proportions are stated in the formulas hereinafter given,but it will be understood that considerable variation is permissiblesince it has been found that an excessive amount of ammonium thiocyanatemay speed up the etching action to such an extent that for printingplates it will produce a harsh efiect due to the rapid removal of themetallic surface particles which, however, may not be deemedobjectionable for certain industrial requirements.

It will therefore be understood that the invention in its basic aspectsis directed to the use of sodium sulphate as the major ingredients withthe addition of a minor amount of ammonium thiocyanate in quantitysuilicient to act as an additional carrying agent without producing asurface reaction regarded as excessive for the use intended and that theaddition of small quantities of ammonium sulphate and zinc sulphate isregarded as a specific refinement to be used in cases where aparticularly high standard of quality in the finished plate is demanded.

The following formulae for the production of '72 gallons of solution aregiven as preferred embodiments of the invention but with theunderstanding that the proportions given are intended to indicate thegeneral order of magnitude of the various amounts and that theproportions may be varied considerably without materially afiecting theresults.

First formula Pounds Sodium sulphate (NazSOO 125 Ammonium thiocyanate(NH4CNS) 3-15 Water in amount sufficient to make approximately 72gallons of the solution Second formula.

Pounds Sodium sulphate (Na-2804) 125 Ammonium sulphate (NH4)2SO4 3-10Zinc sulphate (ZnSOr'lHzOl 3-10 Ammonium thiocyanate (NHiCNS) 3-5 Waterin amount sufllcient to make 72 gals. of

the solution Third formula Pounds Sodium sulphate (NazSOO 125 Ammoniumthiocyanate (NH4CNS) Zinc sulphate (ZNSOr'H-laO) 10 Water in amountsufllcient to make '12 gals. of

the solution Gil surface will under a microscope each present theappearance of a truncated cone enlarged at the base and without theundercutting which is characteristic of acid etching. The conicalformation of the points thus permits the electrolytically-etched plateto serve as a master pattern from which to take reproductions in wax ormetal which will readily separate from the pattern without any marringof the impression which is likely to occur where undercuts are present.

It has been found that a spacing of about 3 7 inches between the zincanode and the cathode with a current of 10 to 30 volts according to thesize of plate to be etched gives highly satisfactory results with any ofthe formulae above given although after usage for a considerable periodof time the accumulation of waste prodnets in the bottom of the tank inthe nature of a sludge a sediment may indicate the desirability ofstepping-up the voltage somewhat to maintain the desired etching speed.A copper cath- Water in amount sufficient to make approximate- 151 '72gallons of the solution and in passing an electric current through thesolution from the anode to the cathode.

2. The method of etching zinc plates which consists in employing as ananode a zinc plate having applied upon its surface the pattern to beetched by the removal of the exposed metal to the desired depth andintroducing the patterned plate in spaced relation to a cathode withinan aqueous solution consisting of the following ingredients inproportions within the general range indicated as follows:

Pounds Sodium sulphate Ammonium thiocyanate 3-5 Ammonium sulphate 3-10Zinc sulphate 3-10 Water in amount suflicient to make approximately 72gallons of the solution and in passing an electric current through thesolution from the anode to the cathode.

ALBERT R. KRECK.

